Signal transmission system



SIGNAL TRANSMISSION/SYSTEM original .Filed Nov. 2s, 1935 Patented Aug. 12,1941

Frank J'. Bingley.

mesnc ents,

ration of Delaware Philadelphia, Pa., assigner, by

to Philco Radio and Tclevision Corporation, Philadelphia, Pa., a corw- Original application November 28, 1935, Serial No.

51,324, now Patent No.

Divided and 5, 193,9, Serial No. 282,935

bei' 5, 1939.

2,171,536, dated Septemthis application July 4 claims. (ci. 11s-44) This invention relates to signal transmission systems and, more particularly, to a transmission system for use in television apparatus. The present application is a division o! copendlng application, Serial No. 51,324, led November 23,' 1935, now Patent No. 2,171,536, granted September 5,

In the said parent application, there is disclosed a complete television system wherein provision is made for transmitting a background control signal to control automatically the background and brightness of the transmitted picture. To this end, there is provided a'signal-combining amplier wherein the several signals including the background control signal, are combined and there is also provided a low impedance transmission line for transmitting the composite signal from the combining amplifier at the television studio to the transmitting station, at which capable of transmitting the background control signal. The present application is directed to the apparatus above mentioned and, more particularly, to that part of the system comprising the said transmission line and the apparatus at each end thereof.

One object of the present invention is to provide a novel signal transmission circuit employing a low impedance transmission line for transmitting a. wide band of frequencies such as thatV encountered in a modern high system.

A more specliic vide a novel signal transmission circuit for transmitting a composite signal of atelevision system, including a background control signal comprising a substantially zero frequency component of the composite signal.

Other objects and features appear hereinafter.

In the accompanying drawing:

Fig. 1 is a diagrammatic illustration of one vform of the invention as applied to a television system; and

Fig. 2 is a fragmentary illustration showing a modified arrangement at the output end of the transmission line.

Referring to Fig. l, there is shown at the left side thereof a signal-combining amplifier which may be located at the television studio as indicated on the drawing. This amplifier comprises a tube V1 energized by a, negative video-or picture signal, a tube V2 energized by a negative backthere is provided a D. C. compensated amplifier object of the invention is to proof the invention will' the horizontal and vertan blanking signals and which may be obtained from the usual synchronizing signal generator.v Since the present invention lsjnot concerned with the specific methods and means of obtaining the several signals involved, it is unnecessary to illustrate or describe them herein. Reference may be had, however, to the above-mentioned parent application for a complete disclosure of such methods and apparatus. v

The outputs of the three tubes V1, Vc and Va are ladded in the common output circuit includinvolved,

ing the resistance R, and the composite signal is used to energize tube V1 by means of the conductive coupling thereto. By virtue of the common load circuit, the negative background control signal and the negative video signal are added together producing a signal including the zero frequency component necessary for automatic background control. The blanking signal may be such as to. maintain the space current of tube V3 at some small value except for the interval during which the scanning beam is returned to a new starting point. During that interval,'the blanking definition television l ground control signal, and tube V3 energized by signal becomes sufilciently positive to increase the current drain of tubeiVs through resistance R enough to bias tube V4 beyond plate current cutoil'.

During the presence of the video signal, tube V5, whichis adapted to be energized'by the synchronizing signals, including both horizontal and vertical signals and whose load circuit is common with that of tube V4, is drawing some constant current which establishes a. predetermined constant voltage drop in the resistance R1. In the absence of a blanking signal, the video signal with the background component is transferred through tube V1 and will appear across the resistance R1. During the blanking interval, tube V4. is biased beyond cut-oil and vconsequently draws no current and, in addition, the synchronizing signal is negative and, therefore, reduces the current drain of tube V5, reducing the voltage across resistance R1, thus establishing the synchronizlng signals 1n the composite signal during the blanking interval with opposite polarity with respect to the video signal.

A low impedance transmission line T is preferably employed to transfer the composite signal from the studio to the apparatus at the transmission station, as illustrated. The resistance R1 constitutes a common load impedance for the tubes V4 and V5 and it will be noted that this impedance is connected in the cathode circuit of the said tubes. The transmission line, which prefer-- ably comprises a coaxial cable,has its input end vconnected to the resistance R1. The surge impedance of the transmission line may be of some small value, say 34 ohms. To avoid reilection, the cable must be terminated at each end by an impedance substantially the same as the surge impedance of the line. Thus. the impedance looking into the terminated line from tubes V4 and V5 will be of the order of 1.7 ohms. This impedance is suiliciently low so as not to cause any serious degenerative eiects in the composite signal ampliiler. It will be noted that the tubes V4 and V5 have their anode-to-cathode impedance in shunt with the external terminating impedance and this must be considered in choosing the proper terminating impedance. Preferably the tubes V4 and V5 should have a high mutual conductance and under these conditions the impedance of the tubes may comprise a, substantial part oi the terminating impedance. Y i

It will be understood that the impedance of the transmission line may vary considerably from the specific value above-mentioned. For example, in one experimental system, the impedance o! the line was about 80 ohms and gave substantially the same results.

The output end of the transmission line is terminated in a resistance R2 which may be coupled to tube Ve by means oi' a blocking battery B1. 'Ihe tube V6 serves to couple the transmission line to a D. C. compensated amplifier, which may take any of the forms disclosed in the abovementioned parent application. The D. C. compensated amplier i'orms the subject of another divisional applicatio Serial No. 282,934, iiled July 5, 1939.

Although a Wide band of frequencies is transmitted through the system, the conductive coupling illustrated in Fig. 1 may be used by virtue of the fact that the capacitive impedance of the blocking battery to ground, even at the highest frequencies, is large compared to the low impedance of the transmission line and, consequently, there will be no attenuation of the high frequencies by reason of this by-pass impedance. t

An alternative form of the coupling at the output end of the transmission line is shown in Fig. 2. In this instance, the resistance R2 takes the form of a cathode load resistor in the output circuit of tube Vs so that therejis provided a cathode drive circuit. T'he choice of the coupling methods illustrated in the two figures will depend upon the desired signal polarity. In the arrangement of Fig. 2, degeneration is practically non-existent due to the low value of the terminating impedance, Further, the voltage drop in this impedance, due to the static space current oi' the tube Va, is of negligible importance.

As mentioned above, while the transmission circuit provided by the invention is particularly useful in a television system, it is not limited to such usage but may be employed in any instance where it may nd useful application. It will be understood too that the invention is not limited to the specic forms illustrated and described. herein for the purpose of disclosure. Further embodiments and modications of the system will be apparent to those skilled in the art and are to be considered within the scope of the invention.

I claim:

1. In a signal transmission circuit, a space discharge device having at least a control element, a cathode and an anode, said device having a high mutual conductance, a signal input circuit connected between said control element and a point of xed potential, a source of electrical energy connected between said anode and said point of fixed potential, a transmission line having its input terminals connected between said cathode and said point of iixed potential,

l means for terminating the output end of said line in its characteristic impedance, said transmission line having a characteristic impedance whici1 is very small compared to the anode-to-cathode impedance of said space discharge device, whereby a small proportion of the voltage of said source is applied to said line, the line conductors are maintained at or near said ixed potential, and the degenerative efiects occasioned by the presence of said line in both the grid-cathode and anode-cathode circuits oi said space discharge device are rendered negligible, a vacuum tube amplifier having its grid-cathode circuit connected to the output terminals of said transmission line, said transmission line providing a conductive coupling means between said space discharge device and said vacuum tube amplifier whereby direct current componentsr of signals may be transmitted therebetween, and a bias battery connected serially in the connection between the grid of said vacuum tube amplier and said transmission line.

2. In a wide band televisionsystem, a television studio, a signal-combining ampliiler at said studio, said ampliiier comprising atleast one space discharge output device having at least a control grid, a cathode and an anode, an input circuit for said device connected between said grid and a point of iixed potential, a source of anode supply voltage connected between said anode and said point of fixed potential, a transmitting station remote from said studio, a transmission line extending between said studio and said transmitting station, said line having its input terminals connected between said cathode and said point of fixed potential, means for tervminating the output end of said line in its characteristic impedance, said transmission line having a characteristic impedance which is very small compared to the anode-to-cathode impedance of said Space discharge device, whereby a small proportion of the voltage of said source is applied to said line, the line conductors are maintained at or near said iixed potential, and the degenerative effects occasioned by the presence of said line in both the grid-cathode and anodecathode circuits oi said space discharge device are rendered negligible, a vacuum tube amplifier at said transmitting station having its gridcathode circuit connected to the output terminals of said transmission line, said transmission line providing a conductive coupling means between said space discharge device and said vacuum tube ampliiier whereby direct current components of signals may be transmitted between said studio and said transmitter, and a bias battery connected serially in the connection between the grid of seid vacuum tube 'amplifier and said transmission line.

3. In a single transmission circuit, a 'space discharge device having at least-a control element, a cathode and an anode, a signal input circuit connected between said control element and a point of substantially fixed potential, a source of said point of ilxed potential, a transmission line having its input terminals connected between said cathode and said point oi dried potential, means i'or terminating the output end oi said line in its characteristic impedance, said transmission line having a characteristic impedance which is very small compared to the anode-to# i cathode impedance of said space discharge de` vice, whereby a small proportion oi the voltage oi said source is applied to said line, the line conductors are maintained at or near said iixed potential,-and the degenerative eilects occasioned by the presence or said line in both the gridcathode and anode cathode circuits of said space discharge device are rendered negligible, a vacuum tube amplifier having its grid-cathode circuit connected to the output terminals of said transmission line, said transmission line providing a conductive coupling means between said space discharge device andvsaid vacuum tube amplier whereby direct current components of signals may be transmitted therebetween, and biasing means for overcoming the small positive voltage applied to the input circuit of said vac'- uum tube amplifier from said source of anode voltage by way of said transmission line.

4. In a signal transmission circuit, a space discharge device having at least a control eleterminating said line in substantially its characteristic impedance, said transmission line having a characteristic impedance which is smallcompared to the anode-to-cathode impedance of said space discharge device, whereby a small proportion of the voltage of said source is applied to v said line, the line conductors are maintained at or near said fixed potential, and the degenerative effects occasioned by the presence of said line in both the grid-cathode and anode-cathode circuits of said space discharge device are rendered negligible, and a vacuum tube amplierhaving its input circuit coupled to the output terminals of said transmission line for deriving signals therefrom.

FRANK J. BINGLEY.` 

